This invention is an improvement in wastewater treatment plants, particularly wastewater treatment plants of the type disclosed in U.S. Pat. No. 5,207,896 granted on May 4, 1993 and U.S. Pat. No. 5,264,120 granted on Nov. 23, 1993, each being in the name of Jan D. Graves, and each being assigned to Norwalk Wastewater Equipment Co., of Norwalk, Ohio.
In wastewater treatment plants of the type disclosed in the aforesaid patents, wastewater is introduced into a pretreatment chamber, flows into an aeration chamber, proceeds to a clarification chamber, flows through the assignee's BIO-KINETIC.RTM. settling system and exits the latter through an effluent discharge pipe. A transfer port is provided at the bottom of a common wall between the aeration chamber and the clarification chamber. Activated sludge settles in a lower portion of the clarification chamber adjacent the transfer port and transient hydraulic currents created in the aeration chamber by for example, a rotating aerator having a plurality of aeration nozzles or aspirator ports, lightly agitate the solids settled in the clarification chamber and they are, in part, mixed into suspension and carried back to the aeration chamber through the transfer ports by these transient hydraulic currents. These transient hydraulic currents are somewhat random and are less than 100% effective at returning settled sludge from the lower portion of the clarification chamber back to the aeration chamber.
Settled sludge is made up of micro-organisms which accomplish the reduction of biological material in the aeration chamber, and it is essential to effectively return them to the aeration chamber. Therefore, having a less than a 100% effective return of the sludge from the clarification chamber to the aeration chamber is not advantageous for many reasons, most notable of which is that these activated sludge solids lose the benefit of being in suspension in the aeration chamber where they obtain the food and oxygen necessary to sustain themselves. Hence, they die or change to a less desirable form. Deprived of oxygen and their normal food source these micro-organisms are subject to other undesirable biological processes. These will naturally occur and can cause a degradation in effluent quality and reduce the efficiency of the overall system in removing pollutants.
As another negative side effect of accumulated sludge in the lower portion of the clarification chamber, it is first noted that aeration accomplishes oxidation of nitrogen as a concurrent but separate step with carbonaceous (organic) oxidation. The oxidized nitrogen (nitrates) remains in the sludge and in solution, but when the activated sludge is allowed to accumulate in the bottom of a clarification chamber for a sufficient time, the dissolved oxygen is consumed and the micro-organisms turn to the nitrate-bound oxygen to breathe. This releases the nitrogen from the nitrate molecule and allows it to form gas bubbles which disperse throughout the "floc" causing it to become buoyant. Once sufficiently buoyant a clump or chunk of the floc will break loose from the sludge layer and float to the surface. This process of creating a bulking of buoyant sludge due to entrapped nitrogen gas bubbles is often referred to as denitrification. The sludge may remain at the surface for a time until further biological processes break it down or the minimal hydraulic currents in the clarification chamber break it up and the gas bubbles are released into the free air. However, while floating on the surface this sludge does not receive sufficient oxygen or food to stay healthy, and it is not returned to the aeration chamber to perform additional biological reduction. The sludge may wash out of the wastewater plant or plug downstream filters and/or settling plates, such as in the BIO-KINETIC.RTM. system heretofore noted. Hence, it is extremely advantageous to preclude the formation of sludge "bulk" or "float."